In practice, Q-switches are used with lasers to generate output laser pulses of short pulse-width and high peak power. Q-switching is a process of first setting the cavity loss in a laser cavity to a high level in order to allow the lasing medium in the cavity to increase population inversion and then momentarily switching the cavity loss to a low level, causing the laser to oscillate, thus producing a higher population inversion level which is then released in the form of a short, high-power laser output pulse. A Q-switch may thus be viewed as an intracavity shutter for the laser. The primary objective of a Q-switch is therefore to create a high-cavity loss by, in essence, blocking one of the cavity mirrors and then to momentarily switch to a low-cavity loss by unblocking the mirror. Many techniques have been developed to achieve this objective.
The simplest and most primitive Q-switching technique is rotating a mirror attached to a highspeed motor shaft. The laser oscillates while the revolving mirror is facing the other mirror of the laser cavity, i.e., while the rotating mirror is in a position to create a low cavity loss condition. A high cavity loss is created at all other times.
An electro-optic Q-switch uses an electro-optic crystal which becomes birefringent when subjected to high electric voltage to create a cavity loss. Although this Q-switching technique is fast and precise, thus providing control over the peak output pulse width independent of output pulse frequency, it has some disadvantages. The electro-optic crystal is subject to optical damage at high intensities, and requires a very fast rising, high voltage pulse driver which tends to be very power consuming, particularly at high (&gt;10 kHz) pulse repetition rates.
Acousto-optic Q-switching techniques use an acousto-optic modulator to produce an rf acoustic wave in order to Bragg-diffract the light out of the cavity. This technique is simple and operates well at a kilohertz repetition rate, but like the revolving mirror Q-switch, it has a very slow opening time and does not work well with high-gain lasers.
A passive Q-switch, such as saturable-absorber Q-switch (also known as dye-cell Q-switch) uses some form of light absorbing material that saturates when the gain inside the cavity exceeds a certain level, at which time the laser begins oscillating.. The dye then quickly drops below its saturation level and oscillation stops. The process automatically repeats to produce successive laser output pulses without the need for any external energy or control. Passive Q-switches are widely used because they are simple, but they have significant limitations, such as pulse-by-pulse amplitude fluctuation and no control over frequency of Q-switching or output pulse width.
Although there have been many advances in the field of Q-switching, there is still a need for a fast, efficient, and stable Q-switching technique that operates at controlled output pulse widths independent of output pulse frequency.